The present invention relates generally to a motor vehicle traction control system and, more particularly, to a part-time transfer case incorporating tractive control means for providing "on-demand" four-wheel drive operation during low traction road conditions.
In general, power transfer mechanisms are used in association with both manual and automatic transmissions for selectively directing power to the wheels of a motor vehicle. One type of conventional power transfer mechanism typically used in four-wheel drive applications is a full-time transfer case which continuously transfers engine power (i.e., torque) to both the front and rear axles. It is common for such full-time transfer cases to provide means for selectively interlocking each of the driven axles for preventing any slippage with respect to each other. It is also common to provide an interaxle center differential assembly for permitting modified torque division and speed variations between the axles so as to prevent potentially damaging torque build-up in the drivetrain. Generally, the torque delivered to the front and rear axles is proportionally divided via the interaxle differential assembly installed within the full-time transfer case.
Another type of power transfer mechanism is a part-time transfer case adapted to normally operate in a two-wheel drive mode and which requires the vehicle operator to intentionally and deliberately shift into a four-wheel drive mode. Still another type of power transfer mechanism is an "on-demand" four-wheel drive apparatus, such as a viscous transmission unit, operable for automatically transferring power to the non-driven axle when traction is lost at the drive axle.
With the advent of increased consumer popularity in four-wheel drive passenger cars and sport/utility vehicles, full-time and part-time transfer cases as well as "on-demand" transmission devices are being utilized in a plethora of vehicular driveline applications. One example of a full-time power transfer system is disclosed in SAE Technical Paper No. 892538 entitled "Electronic Transfer Case For 1990 Aerostar Electronic Four-Wheel Drive" by John Richardson and Wes Dick. The full-time transfer case disclosed is incorporated into an all-wheel drive vehicle that is primarily intended for road use as a family car and not as an off-road recreational or sport utility vehicle. This full-time transfer case employs a clutch assembly applied across the center interaxle differential assembly for modifying the torque split in relation to the available traction at each axle. More particularly, the full-time transfer case is a single-speed power transfer apparatus which does not provide a two-wheel drive mode, a low gear four-wheel drive mode nor means for positively over-riding (i.e., locking out) the interaxle differential assembly.
As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between a two-wheel drive mode during normal road conditions and a four-wheel drive mode best suited for operation under adverse road conditions. However, conventional part-time transfer cases of the type used in off-road, sport and utility vehicles do not provide an "on-demand" four-wheel drive feature which can be effectively "over-ridden" upon the vehicle operator selectively shifting the transfer case into the four-wheel drive mode to permit off-road and/or recreational use in a two-speed traction control system.